The present invention relates to a short circuit protection circuit for a relay arranged in an electronic device that is installed in a vehicle.
A relay drive circuit for an in-vehicle electronic device installed in a vehicle supplies a relay (load) with drive current based on an input control signal. When a short circuit occurs at the output side of the relay drive circuit, excessive short circuit current may flow to the relay drive circuit. This may damage the relay drive circuit.
Japanese Laid-Open Patent Publication No. 2001-25150 describes a short circuit protection circuit including a large current fuse located between a power supply and a relay drive circuit. The fuse melts and breaks when a short circuit occurs. Further, the short circuit protection circuit monitors the current flowing to the relay drive circuit and restricts the amount of the current to protect the relay drive circuit from damages caused by a short circuit.
One type of a short circuit protection circuit forcibly stops the output operation of a relay drive circuit when a short circuit occurs and prevents short circuit current from flowing to the relay drive circuit. After a predetermined time elapses from when the short circuit occurs, the short circuit transmits a recovery signal to the relay drive circuit in predetermined time intervals to recover the output operation of the relay drive circuit.
With reference to the timing chart of FIG. 1, in such a short circuit protection circuit, after predetermined time ΔTc3 elapses from when a short circuit occurs, recovery signals Re1 to Ren (where n is a natural number) are respectively transmitted in a predetermined time interval ΔT at times T1 to Tn (ΔT=Tk+1−Tk, where k is a natural number; 1≦k≦n, where n is a natural number). This recovers the relay drive circuit to an output operation state.
If the recovery signals Re1 to Ren are transmitted (the recovery operations for recovering the relay drive circuit) too often, excessive short circuit current flows to the relay drive circuit when the operation of the relay drive circuit is temporarily recovered by the recovery signals Re1 to Ren. As a result, heat generated by the short circuit current is accumulated as time elapses. This may damage the relay drive circuit (transistors etc.). On the other hand, if the recovery signals Re1 to Ren are transmitted less frequently, recovery of the relay drive circuit from the short circuit state is delayed. This prolongs the time in which output from the relay drive circuit is stopped.
In FIG. 1, at time T2, the recovery signal Re2 is transmitted to the relay drive circuit to recover the output operation of the relay drive circuit. Further, the time ratio of the time [min] required from cancellation of the short circuit to recovery of the output operation relative to the time of the short circuit state [min] is 50%. In this case, the transmission frequency (number of transmissions) of the recovery signals Re1 to Ren is two and small. Further, there is no damage to the relay drive circuit. However, the time ratio is 50% and relatively long. Such a result is not satisfactory. In the example shown in FIG. 1, it is assumed that the relay drive circuit is damaged by the accumulation of heat generated by short circuit current when the recovery signal is transmitted to the relay drive circuit six times or more.
The relay drive circuit may be manually operated by a user of the vehicle (driver etc.) to recover the relay drive circuit from a short circuit state to an output operation state within a short period while preventing damage of the relay drive circuit. However, this would be inconvenient to the user.